Three-dimensional magnetite replicas of pollen particles with tailorable and predictable multimodal adhesion

The ability to synthesize large quantities of 3-D microparticles with tunable adhesion is critically important for a variety of mature and emerging technologies, such as for paints, inks, chemical/water purification, drug delivery, cell manipulation, and assembly of hierarchical structures. Nature provides impressive examples of sustainable, complex-shaped microparticles with chemistries and structures tailored for adhesion, among the most common of which are pollen grains. We have recently used a surface sol-gel (SSG) coating process to generate iron oxide replicas of sunflower pollen grains. While these replicas exhibited multimodal adhesion, the tailorability and predictability of such adhesion was not examined. In the present paper, the layer-by-layer SSG process has been used to carefully adjust the amount of iron oxide deposited onto the pollen grains. Controlled-atmosphere thermal treatments then yielded freestanding replicas with tailored hematite (alpha-Fe2O3) or magnetite (Fe3O4) contents. The 3-D morphology of the starting pollen was well-preserved in the all-oxide replicas, and the shrinkage upon firing could be controlled by increasing the number of Fe-O-bearing layers deposited on the pollen. While the short-range van der Waals (VDW) adhesion of the oxide replicas to a variety of surfaces was lower than for the larger starting pollen grains, this difference was not due to shrinkage of the replicas. Analyses with a simple Hamaker model indicated that VDW adhesion of the oxide replicas was governed by the contact of oxide nanocrystals located on the spine tips (as opposed to the curvature of the entire spine tip). The longer-range attraction to a magnetic substrate could be tailored independently of the short-range VDW attraction by controlling the magnetite content of the replicas, and a simple and effective model for describing such magnetic attraction was developed. This work demonstrates that sustainable pollen microparticles can be converted into high-fidelity 3-D oxide replicas with predictable and tailorable multimodal adhesion.